trustzone-sdk: add ring 0.17.14 for optee

[DemesneGH]

This PR adds the OP-TEE target to the ring source code, enabling it to be used in TrustZone SDK TAs with std support.
An example demonstrating how to use this crate is provided here: DemesneGH/rust-optee-trustzone-sdk@2f26866

After this PR is merged, I will update the link of ring in Cargo.toml accordingly: DemesneGH/rust-optee-trustzone-sdk@2f26866#diff-f364a71ab8e7d11e3fae1e6ccf83fd0fe4254583945c71924405918dbe51dc59

Next, I will open a PR to the TrustZone SDK to integrate the ring ecdsa example.

[DemesneGH]

Since there're all third-party crates in this repo, how to handle the license?

[ivila]

I don't get it, why not elliptic-curves from RustCrypto, it supports no_std, save us from porting codes like this.

[DemesneGH]

I don't get it, why not elliptic-curves from RustCrypto, it supports no_std, save us from porting codes like this.

elliptic-curves is indeed a good crate, it supports no-std and fits the our need of crypto. But ring is the dependency of rustls, and rustls is the dependency of our tls examples. Our final goal is to provide a new rustls version which eliminate the security warnings https://github.com/apache/incubator-teaclave-trustzone-sdk/security/dependabot. To achieve that, we need to first port ring, then update rustls accordingly.

[DemesneGH]

I see the new version of rustls supports no-std: rustls/rustls#157 (comment). I will try if it works for our case.
For this PR, I will fix the license issue then it's ready to merge.
For the ring ecdsa examples, it is just a test for the ported ring, maybe it should not be included in the trustzone sdk examples, let's keep it pending for now.
Please feel free to share any suggestions about this, thanks!